The Japanese grill them, the Dutch smoke them, while the English pack them into little pots of jelly or bake them in fragrant pies. Why then do many Australians shrink in horror at the idea of a meal prepared from eel?

Snake-like and slimy in appearance, freshwater eels are not the most appealing of animals. They scare swimmers who know they are lurking beneath rocks in your local river or creek, and when it rains, they emerge from the water to weave their way stealthily across the ground and up dam walls in their efforts to migrate upstream.

Growing up to two metres long and weighing up to 20kg, mature adult eels dwarf all other creatures living in the waterways, as they grow fat on the crustaceans, dragonfly larvae and small fish they catch and grind up with their plate-like teeth.

But there is a secret side to eels that few have studied and none have seen first hand. They are catadramous: beginning life far out in the deep ocean, migrating to the continent to live their adult life in freshwater rivers and lakes, then finally returning once again to somewhere unknown in the deep ocean to breed and die.

It is a remarkable story of endurance in the face of extraordinary obstacles, great physical hardships and, finally, death at the bottom of a cold dark sea, right on the spot where they were born, but thousands of kilometres from where they lived out most of their life.

Spot the difference

Freshwater eels, along with morays and congers belong to the group known as the bony fishes (other main groups of fish include sharks and rays). But freshwater eels are so different to moray and conger eels that they have been put in a separate family, the Anguillidae. This family contains 15 species, two of which, Anguilla reinhardtii (longfinned eel), and A. australis (shortfinned eel) are found in Australia.

Freshwater eels are long and muscular with a skin which is brown or yellow in young fish and gradually turns mottled (longfinned eel) or darkish olive (shortfinned eel) as the fish matures. Adults preparing for their seaward migration gradually turn silver.

Longfinned eels are the more tropical species, while shortfinned eels tend to live in more temperate areas, although both can be found up and down the east coast of Australia (including Tasmania and South Australia) and as far as the Bass Strait Islands, New Zealand, Lord Howe Island, and New Caledonia. [View disribution map]

While the two eels' territories overlap, they are easy to tell apart. The top (dorsal) and bottom (anal) fins of the shortfinned eel begin at about the same point on the body, and a fair way back. The dorsal fin of the longfinned eel, on the other hand, starts well in front of the anal fin, and this species has a much more spotted and blotched pattern across its body. It prefers flowing rivers and creeks more than the calm ponds and lakes the shortfinned tends to inhabit.

Winter fare

The closure of autumn and onset of winter brings the first year's batch of tiny new eels along the coast and into the rivers. Already these eels have travelled thousands of kilometres from hatching grounds, deep in the Coral Sea, near New Caledonia.

After hatching eels look not at all like their mature selves. They begin life as a clear, gelatinous leaf-shaped larvae called a leptocephalus ('narrow head'). For months these tiny creatures float along down the east coast of Australia, swept along in the same currents as millions of their planktonic companions.

They have an excellent sense of smell, and can detect minute amounts of chemicals in the water. The rich cocktail of organic molecules that flows from every river is eventually detected by the tiny eels out to sea, guiding them in towards the estuaries and away from their marine existence.

The leptocephali metamorphose into glass eels, creatures similar in appearance to an adult eel except virtually transparent. On a new or full moon when the tide is running in, glass eels can sometimes be seen moving upstream in their thousands, particularly in Queensland's Albert River and in Victoria's Snowy River in East Gippsland. This phenomenon, called an eel run or eel fare, is when most of the fishing is done, because the eels are easier to catch when they are schooling. [See eel life-cycle]

An industry based around fishing for glass eels has developed in the eastern states. Glass eels are a highly prized food item in Asia, selling for $1000 a kilogram in Japan, but are also collected by eel farmers to grow up to adult size.

Fact file:

When: The end of autumn and onset of winter brings the first year's batch of tiny new eels along the coast and into the rivers.

Where: East coast of Australia

Other info: Sexing the Eel - Unlike many animals, it is believed that the sex of an eel is determined environmentally, rather than genetically, says Lachlan McKinnon, a senior scientist with Primary Industries Research Victoria. Small eels are sexless, but at some point, factors such as population density and competition cause hormonal changes that lead to eels becoming either male or female. Although their gender has been resolved, eels may not become ready for breeding for anything up to 50 years.

Marathon eels

The going is never easy when you're an eel. Although now large enough to fend off predators, a young 30cm long eel must contend with major obstacles along the river such as dams and the spinning turbines of hydro power stations. Many eels become instant fish mince as they pass through the huge turning blades. But sometimes river regulators take pity on the eels: five years ago, the sudden migration of eels down the Patea River in New Zealand prompted the local hydro station to cease operations for several days to allow them safe passage.

Dam walls and weirs pose another daunting barrier to eels. But they have a secret weapon no other fish possess: a slimy skin covered with tiny scales which allows them to 'breathe' on land, obtaining some oxygen by direct diffusion through the skin. As long as it's a dewy or rainy night, eels can leave the water and wriggle across land and even straight up dam walls. Recently some dams have been covered with Astroturf to make it easier for the eels to climb, and fish ladders have been installed in several rivers to aid their migration.

Once she arrives in the upper reaches of the river around springtime, an eel may remain there for decades, perhaps up to 50 years. Despite their cryptic ways, eels play a very important role in these ecosystems because of the large amount of food they eat, says McKinnon. "They are the top carnivore - if they were removed from the system there would be a shift in the biodiversity."

As the autumn rains swell the headwaters of Australia's easterly rivers, the mature eels who have languished there for decades start to move downstream towards the coast. No-one knows what triggers migration, but it may be a combination of low atmospheric pressure, high rainfall and a new moon. If the right conditions don't happen, some eels become landlocked for the rest of their lives, growing to enormous size. But McKinnon says even large eels are relatively harmless: "most people have a primitive fear of eels because they look like snakes. If you swam with them, they might be curious but wouldn't bite unless you grabbed them. The teeth are very short and formed into plates, so the worst you'd get is a V-shaped welt. When you work with them, they all have personalities. They are quite mellow."

Eel season

As the eel begins her final journey downstream, a number of strange changes begin to take place. Already her skin has begun to darken and thicken, and the fins and eyes have grown larger. As she moves towards the sea, irreversible changes occur: the anus constricts to reduce water loss and the stomach degenerates, freeing up vital resources for beefing up the reproductive organs.

This would have been the time of year in pre-European Australian cultures when the local people would catch eels as they went about their migration - autumn was known as eel season and a time of plentiful food. There is vivid testament to these times at Lake Condah, in western Victoria, where the remains of stone fish traps perhaps 8000 years old indicate that the local people took advantage of the migration cycle of eels. They trapped them and then smoked them in the hollows of nearby trees for eating and trading.

Although by the time the eels reach the sea they have already travelled hundreds of kilometres from their mountain home, their journey has only just begun. Now they must turn north, and travel back to their breeding ground up to several thousand kilometres away somewhere in the South Pacific.

No-one knows how they find their way to their spawning grounds. One idea is that eels use magnetic cues or even that they pop up to the surface and navigate by the stars. Although they do swim against the southerly flowing East Australian Current as a guide, how they find their way to the exact spawning grounds is still a mystery.

Here, the eels, ripe with up to five million eggs spawn and then - although again, no-one knows for certain - die from exhaustion.

Why do eels go to all this trouble, when many other fish live out their lives quite happily in one river? Perhaps it's because freshwater eels may be descended from fish which, millions of years ago, lived out their entire life in the deep tropical ocean. Perhaps the echo of their far off ancestors just keeps calling the eels back to their oceanic origins.

Credits

Special thanks to:Special thanks to Dr Lachlan McKinnon, Primary Industries Research Victoria.Dr Bruce Pease, NSW Fisheries for help in preparing this article and for supplying images.Thanks also to Cedric Briand for use of the image of glass eels.